Absorption cooling cycles offer great potential to utilize waste or low-grade heat but have been hampered by low efficiency, high cost, and space requirements compared to compression-based refrigeration. Current commercial absorption refrigeration devices have a limited market demand due to high cost and low efficiency.
As concerns about global warming continue to grow, technologies to reduce energy requirements have received greater attention.
An absorption refrigerator is similar to a regular compressor refrigerator in that the cooling takes place by evaporating a liquid with a very low boiling point. When the liquid evaporates or boils, it removes heat with it, and it will continue to do so either until the liquid is all boiled, or until everything has become so cold that the boiling point has been reached. The only significant difference between absorption and compressor refrigerators is how the refrigerant gas is converted back into a liquid for reuse.
A compressor refrigerator uses a compressor to increase pressure on the gas, causing it to convert back to liquid again after cooling. An absorption refrigerator uses two components which have a strong affinity to combine, to absorb the refrigerant gas into to the absorbing liquid. This process can require no moving parts and be powered only by heat.
Absorption refrigeration cycle was the first widely used refrigeration system. In the days before rural electrification, absorption refrigerators were widely used. These designs operated by using a high temperature gas burner to supply energy for movement of the working fluids using the percolation effect and then using the high-temperature exhaust to induce a draft for air movement. Today, many absorption refrigerators and cooling units still use combustion-supplied high-grade heat to provide the energy needed to drive the cooling process.
Fluorocarbon refrigerants and inexpensive sealed compressors made compression cycle refrigeration the predominant method after World War II. Nevertheless, absorption systems have continued to develop. Most of these modern absorption units are designed for static installations and are equipped with pumps, heat exchangers and cooling fans to improve efficiency. Progress has been incremental and unable to successfully compete with compressor driven refrigeration except in niche markets.
One of the challenges faced is that both of the main refrigeration mixtures have significant problems. The classic ammonia-water system of the original concept provides low evaporator temperatures, but can cause serious injury and death if leaks occur and people come in contact with large amounts of gaseous ammonia. This danger encouraged development of systems using lithium bromide-water as the refrigeration fluid mixture. Lithium bromide also becomes a crystalline salt when the concentration of water gets to low, requiring care and controls to maintain liquid state. It uses water as the refrigerant requiring very low pressures in the evaporator and limiting the evaporator temperature to near 50 degrees F.
One of the greatest limitations on absorption refrigeration cycles is the difficulty of handling either high pressures in an ammonia cycle or the near vacuum of the lithium bromide cycle. Pumps and heat exchanger have incorporated design improvements. Many existing systems are not optimizing heat exchange, especially when high ambient temperatures cause low temperature differentials, causing efficiency to drop.
A low heat exchange temperature differential requires significant dwell time to transfer maximum heat energy, but most refrigeration units operate on fixed mass flow rate designs. To maintain the required pressure differential between the desorber and absorber requires pumps and throttling valves in commercial systems, both of which reduce efficiency. There is a need for an improved system that offers a solution to the above-identified issues.